The cracking of a crucible can cause heavy damage to the system in induction furnaces, and in addition can endanger personnel. Various systems are already being used for the timely supply of information regarding threatening breaking out of melt.
Usually in the simplest case the contact of melt with the coil is indicated through ground fault. Warning systems installed in front of the coil, for example on an electrically insulating refractory liner applied to the coil, create a warning prior to the running out melt contacting the coil. The control and design of the warning systems exist in various forms. Simple systems measure the resistance in the circuit crucible—warning system—gauge. For this purpose the gauge has to be connected by means of a contact, in most cases a stainless steel flag embedded into the bottom refractory, to the crucible surrounded by the induction coil. When the crucible cracks, a melt tongue run out penetrates the refractory liner and contacts the warning system. The prior very large electrical resistance of the refractory liner decreases, due to the contact, practically to zero. The monitoring system indicates this optically or acoustically and switches the furnace off.
The disadvantage of this system is that when the contact in the circuit is lost a change is not displayed, for example, due to a cable disruption caused by shrinkage of the crucible or due to oxidation of the stainless steel flag, which results in an increase of the transfer resistance at the flag. This results in the monitoring device becoming ineffective since the resistance display remains at infinite even when a melt tongue touches the warning system.
Attempts to guarantee an earlier recognition of a crack in the crucible with the breaking out of melt are known from the publication of Hopf, Giesserei 89 (2002), No. 1, Pages 36–42. The residual wall thickness is thereby measured by the temperature based on the temperature dependency of the specific electric resistance of refractory material, and is utilized to evaluate the localized wear of the furnace wall thickness. Sensors are for this purpose installed in a refractory construction, which sensors are made of two wire-shaped, comb-like electrodes which are embedded into a flexible mica or ceramic material. If the temperature now increases at one point of the sensor, then the specific electric resistance of the ceramic is reduced at this point. This resistance change between the two electrodes is detected by a measuring device and is evaluated.
However, this works only to a limited degree in particular in furnaces with electric or clay-graphite crucibles since the explicit temperature dependency of the resistance does not exist here.